This invention relates generally to array antennas used in radar systems and feed networks for such antennas and more particularly to antennas of such type which are adapted to provide monopulse tracking.
As is known in the art, array antennas, such as cylindrical array antennas, have been suggested for use in many applications of radar requiring wide scan angle coverage, such as in known airborne "multimode" radar systems. In such a type of array antenna a plurality of antenna elements circumferentially disposed on the surface of a cylinder corresponding generally to a portion of the fuselage of an aircraft are coupled to a transmitter/receiver through a feed network. Various types of feed networks generally used in such applications are described in an article entitled "A Survey of Circular Symmetric Arrays" by J. H. Provencher published in the book "Phased Array Antennas" edited by Drs. A. A. Olinir and G. H. Knittel, published in 1972 by Artech House, Inc., 610 Washington Street, Dedham, Mass. 02026. The feed networks shown in the just-cited reference include various kinds of switching arrangements to direct radio frequency energy to selected sectors of the antenna array so that a resulting beam has a desired circumferential position. It is obviously desirable that the particular feed network and the switching arrangement used therewith include a minimum number of switching elements in order that power loss, phase error and amplitude error within the antenna be, in turn, reduced to a minimum. While the feed networks described in the above mentioned article have been found adequate in many applications, such networks require the use of a relatively large number of switching elements. The concomitant losses and errors inherent in such networks then militate against their use in applications in which beam shape and low sidelobe levels of a beam are of primary importance.
The shortcomings of known feed networks for the antenna elements of an array antenna are especially evident when a cylindrical or frusto-conical shaped array is desired to provide monopulse tracking of targets within a relatively wide field. It is generally desirable with any such array that the gain of the antenna be maximized for both circumferential and axial scans.
In order to maximize the gain of such an antenna array in the axial direction, it is desirable that the radiating elements be configured so that the E field (i.e. the polarization) of the radio frequency energy in the "near" field is codirectional with the longitudinal axis of the array. It follows, then, that when axial scanning is effected (i.e. when the beam is scanned in a plane containing the longitudinal axis of the array) the "far" electric field may be considered as having a component orthogonal to the radiating face of the array. This component in turn has two components, one orthogonal to a selected antenna "plane" (i.e. any plane orthogonal to the normal to the array at the center of the beam) and the other being parallel to such antenna "plane." Unfortunately, then, if any of the feed networks of any of the antenna arrays illustrated in the cited reference are adapted for use in either a cylindrical or a frusto-conical array, undesirable cross-polarization components exist between radiating elements. That is, with known feed networks the "far" electric field component parallel to the "plane" (i.e. the cross-polarization components) generally seriously degrades the antenna monopulse "sum" and "difference" patterns at large axial scan angles. Such pattern degradation is sometimes referred to as "null filling" because the cross-polarization components tend to form a "sum type" pattern for the monopulse "difference" patterns and also tend to form a "difference type" pattern for the monopulse "sum" pattern at the center of such patterns.